1. Field of the Invention
The present invention relates to a lens system with a rear stop diaphragm.
2. Description of the Prior Art
In most ordinary photographic lenses, a stop diaphragm is provided between selected lenses within the lens system. It is, however, necessary to dispose a stop diaphragm behind the lenses, that is, in the body of the camera, in order to facilitate the operational construction of the camera mechanism. In such a lens system with a rear stop diaphragm, generally the aperture of a first lens group increases and it is therefore difficult to produce a lens system where various aberrations are well compensated for or corrected.
Ordinary telephoto lenses are so contructed that front lens groups thereof have total positive lens powers while rear lens groups thereof have total negative lens powers. In order to miniaturize such lens systems, in general, the respective powers of the front and rear lens groups are increased and in addition, the space between the front and rear lens groups is increased. That is, the physical length from the rear lens group to the focussed image surface or the "backfocus" is shortened to thereby reduce the overall length from a first surface of the front lens group to the image surface. However, in most lens systems having rear stop diaphragms it is considered that possible minimum backfocuses have already be attained. In this case, the increase of the power of each lens group is substantially the same as the decrease of the space between the front and rear lens groups. Moreover, the edge thickness of each positive lens is too small. For this reason, as a practical matter, the central thickness of the positive lens must be increased resulting in elongation of the front lens group. For a beam of light passing through the most peripheral portion of the first lens, a higher incident height is required for the front lens group. This leads to difficulties in miniaturizing the physical size of the lens system.